This invention relates generally to tethered balloons, and, more particularly, to a variable lift inflatable airfoil structure for use therewith.
In rigid airfoils such as wings and horizontal stabilizers of conventional aircraft, variable lift is provided by hinged extensions of the airfoil in which the angle of articulation is mechanically adjusted to provide the lift characteristics desired. These airfoils are commonly called ailerons and elevators.
In lighter than air aircraft, such as ballons, however, since they are generally of an inflatable fabric construction it is not possible to provide these crafts with "conventional" variable airfoil structures because: (1) the high forces required to drive the small moment arms of the movable part of the airfoil cannot be supported by the soft inflatable structure of both the fixed and movable parts, and (2) the use of stiff metallic structure to accomplish (2) purposes set forth hereinabove would be too heavy to be practical in such lighter than air aircraft and would also be destructive to the soft inflated structure.
In dirigibles some elevators and rudders have been utilized by the attachment thereof to the reinforced section of the dirigible. This is possible because these elements in the form of fins are small, are not inflated structures and basically the forces developed are also small. Dirigibles, however, basically involve a different stability and flight regime than lighter than air tethered balloons. In addition, dirigibles are pitch-neutral and are in free flight, whereas a tethered balloon is pitch-positive and restrained similar to a kite. It would be extremely desirable to utilize variable lift airfoil structures on tethered balloons. Unfortunately, the airfoil structures heretofore discussed would cause greater problems to the tethered balloon flight than would be solved by their addition thereto.
Generally tethered balloons utilize fins which develop zero lift at zero angle-of-attack because the airfoil structures associated therewith are symmetrical. The symmetrical airfoil structure at zero incidence has equal aerodynamic pressure on both upper and lower surfaces thereof and therefore does not develop any lift at zero angle-of-attack. Conversely, if the airfoil were made unsymmetric in section and if the asymmetry were of a desirable configuration, such a structure would only develop lift at zero-angle-of-attack. Such a fixed structure, however, although effective under certain circumstances, would not be particularly useful because the lift characteristic would remain fixed. Furthermore, such an unsymmetrical airfoil utilizes conventional chordwise ribs in its construction, which in itself leads to two additional problems: (1) because of Pascal's Law the shape would be as desired only at the rib sections and would be nearly symmetrical at stations midway between ribs unless an intolerably large number of ribs were used and (2) inflated airfoils which are shape-determined by chordwise ribs have poor shear stability and therefore inferior stiffness to cantilever bending.
It is therefore obvious that there is a great need for variable lift capability in tethered balloon design. However, as set forth above utilization of conventional variable lift design results in greater drawbacks to the overall balloon efficiency than the benefits derived therefrom.